The present invention relates to a delay stage for a semiconductor device. The invention further relates to a ring oscillator, a PLL-circuit and a method for operating the delay stage.
A typical delay circuit delivers an output pulse at some predetermined delay time after receiving an input pulse. The predetermined delay period, that is the period between the input and output pulses, must be accurate and repeatable. Those delay circuits comprise a delay stage especially for use in a semi-conductor device, such as an oscillator.
There are numerous electronic circuit applications where, for proper operation, it is necessary to provide precise timing or synchronization of one portion of the circuit with another one. Such timing is conveniently provided by an oscillator whose frequency is sufficiently accurate for the requirements of the circuit being timed or synchronized. Depending on the degree of accuracy required, an oscillator may be very simple where the frequency range can have wide latitude on the one hand or relatively complex where a high degree of accuracy in frequency is required.
Various kinds of oscillators have been used for onboard timing of other circuitry. One kind of oscillator which lends itself particularly well to implementations in CMOS technology is a ring oscillator. A ring oscillator is an electronic oscillator which oscillates independently and does not need any external components, such as capacitors or coils, which in other oscillators are used to accurately set the frequency of operation. Instead of these capacitor or inductor tuned circuits, a ring oscillator has number of simple inverting stages. These stages of a ring oscillator are commonly also referred to as delay stages or delay cells. The frequency of operation of a ring oscillator is determined by the speed of progression of a switching event from one delay stage to another around the ring of the ring oscillator and by the number of delay stages.
A conventional ring oscillator may have its own frequency of operation within a very wide range of frequencies. However, by way of example, in a CMOS based ring oscillator currently being manufactured as part of a CMOS IC chip, the difference in frequency of the oscillator of one chip from the frequency of the oscillator of another supposedly identical chip can be rather great. This wide range of frequency of operation and thus the performance of ring oscillator circuits subject to at least three significant basic tolerances: supply voltage fluctuations, temperature variations, and basic process variations from chip to chip. Therefore, where precise timing is required, e. g. frequency accuracy, to within a few percent, such wide frequency ranges are not or not throughout acceptable. Because of the above mentioned variations, ring oscillators have generally been deemed overly unstable and subject to frequency drift.
If a commonly known ring oscillator is used, for example, in a PLL-circuit (PLL: Phase Locked-Loop). The large variation in the oscillator gain causes an unwanted large variation in the PLL-bandwidth. A frequency divider of the PLL-circuit which is connected to an output of the ring oscillator must therefore be designed for the highest possible oscillation frequency of the ring oscillator. However, frequency dividers for high frequencies usually consume more power than frequency dividers which are designed for lower frequencies.